Acid resistance of calcium aluminate cement in concrete sewer pipe mixtures

There is increasing interest in the use of alternative binders for the manufacture of concrete sewer pipes. These binders could be used either in concrete to form the structural shell of a sewer pipe, or to produce acid-resistant linings. Calcium aluminate cement has many attractive benefits for use as a lining material, for instance rapid hydration and strength gain, good bonding properties with the substrate, and an expected improved performance in the presence of acidic conditions. Current work at the University of Cape Town is concerned with evaluating calcium aluminate cement (CAC) binders in comparison with a range of Portland cement (PC) binders and cement enhancers. Experimental approaches have involved testing these binders under conditions of strong mineral acid attack, and under conditions of simulated biological acid generation. The paper presents results of these investigations, and seeks to provide explanations for the relative performance of the different binders. It is shown that under conditions of strong mineral acid attack, CAC binders do not necessarily perform better than PC binders. However, a CAC/ synthetic Alag aggregate mixture showed excellent acid resistance. Initial observations on sewer test pipes also show CAC mixtures to be resisting acid attack better than PC mixtures; and this is ascribed to their ability to suppress bacterial activity.